I made some excellent (small step) progress on my airplane build this past week.
Before I get into details, I want to share a bit of scrounging advice. Don't ever be afraid to ask around when you are looking for something, be it materials or tools.
While building, Ron and I often get to talking about ways to save on costs. One of the things that costs a bunch of money when getting it done by others is powder coating parts. Powder coating is a dry finishing process that gives various materials a durable coating that can be much tougher than paint alone. It's particularly good on non load bearing parts that may be handled regularly or exposed to friction. Control columns and rudder pedals come to mind.
Powder coatings are based on polymer resin systems, combined with curratives, pigments, and other additives and ground to a fine powder. A process called electrostatic spray deposition (ESD) is typically used to apply the resin to the metal substrate. The process uses a spray gun which applies an electrostatic charge to the powder particles which are attracted to the grounded part. After application of the powder coating, the parts enter a curing oven where, with the addition of heat, the coating chemically reacts to produce long molecular chains, resulting in high cross-link density.
That's the long way of saying "it sprays on and sticks really well after being cured in the oven".... ha!
Ron and I both figure the majority of the parts we might want powder coated should be able to be done ourselves. Ron has a source for the powder coating gun and resins, we just need an oven. Baking resins can generate a fair amount of unpleasant fumes, so we won't be using the kitchen!
I've been real fortunate over the course of the last few years to have several people I know come to me with leads on "airplane stuff" and I owe a bunch of that to talking to everyone I know about my project and plans. Opinions regarding my sanity range from "wow, that's cool" to "you are bat-s%$t crazy dude!" However, even if the vast majority consider me closer to the slightly crazy side of the scale, they do come to me when they hear of something.
In this case, when I mentioned that we were seeking an oven, Brenda noticed a Facebook post from a friend of a friend who was remodeling their kitchen. Turns out they were giving away a built in Jen-Air oven! Free! Brenda messaged them, I hopped in the truck and 10 minutes later, it was in our possession. We really don't need the stove top portion for baking parts so this is perfect:
We'll build a simple stand and wire it for power. It will require some calibration tests to ensure the temperature settings are accurate as they need to be for the powder coating. Not every oven is created equally as far as accuracy is concerned and oven temperature can drift as much as 25 to 50 degrees over time.
As for my airplane, I started to put the templates I made to use and traced out my first parts with them.
They worked real good. A thick Sharpie marker leaves a good line for rough cutting:
Before making the rough cuts of individual pieces from the sheet, now is the time to drill the corner relief holes where reuired. Here are some that I remembered to drill before cutting them out. Much easier to do this before hand I've learned!
Once the parts are rough cut out (thicker pieces on the bandsaw), further fine cuts are made using hand tools. By always leaving a bit of the thick marker line, we can see where the part will be trimmed down with the grinder, a file or hand sanding when taking of the burrs.
I made several parts over a couple of hours:
I took a good idea from Ron and taped the template to the parts when they were done. That way I don't have to write the part numbers on the aluminum. These completed parts will be stored until I need them later. I'm keeping a massive spreadsheet to track parts made, where they are stored and what inventory of materials I have on hand:
I know I have a TON of parts still to make, some simple, some complex.... but there is something so motivating about making these first parts for my 750 that makes me want to be in the shop full time. Unfortunately without spending at least some of my waking hours at my paying job, I can't afford the materials to make parts, so I guess I'll have to get back to the shop when I can.
Next up, further repairs to the 701 wing and I'll finish the sub assembly parts I need for the tail group on my 750!
As I work away on the repair of the 701 wing I'm caught in a bit of a conundrum.
The spar cap repair I'm ready to rivet is all fit and ready to go, but in order to solid rivet the spar cap inside the nose skin, I need to take that nose skin off in order to have the room required for the rivet gun. Not a huge deal and not difficult to do, but as with everything else in this repair, we're scared to discover more of the original builders follies.
The other side of the coin however is do we want to take that chance? Forego opening the nose skin section and we might miss something important, but we can't leave it in place if we expect to complete the spar cap repair. Eventually the MDRA inspector will need to see inside the wings anyhow, so off it comes.
First steps, drill out all the nose skin rivets on the bottom edge and rib noses:
Gently unroll the nose skin which can very easily be creased when not secure:
Thankfully it came apart very easy. It actually looks decent and proper inside. Some minor issues with rivet lines on the nose ribs, but certainly better than what we've seen so far.
With the nose open, I finished solid riveting the spar root doubler. Nice to see this part close to being done - really happy how it turned out, the solid rivets are not too difficult to set:
I also finish riveted the spar cap repair. It too turned out real nice:
While all this has been going on, I've been working on printing and cutting out the templates I need to make my forming blocks and shaped aluminum blanks for my 750 build. It was lot of work but now I can start cutting and bending metal for my project!
Next up, continue the 701 wing repair (almost there!) and start tracing out the parts for my 750 horizontal stabilizer.
One of the new skills I'm learning is how to "buck" rivets.
Bucked rivets, also referred to as driven or solid rivets, have been used as the primary fastener type in the construction of aircraft for decades. These solid rivets are light-weight, strong and inexpensive fasteners. That’s why they and pulled rivets were chosen for aluminum aircraft construction.
A bucked rivet is a round fastener that attaches two or more pieces of metal together. The rivet is driven by a pneumatic rivet "gun" with an attached rivet "set" shaped according to the shape of the manufactured head of the rivet. The rivet's "tail" (blunt end) is backed up by a "bucking bar" that acts as an anvil while the rivet gun and set are acting as a hammer.
As rivets are driven the tail of the rivet is transformed (technically called “upset”) and two things are accomplished. First, the rivet shortens in length and the exposed tail bulges outward to morph into what is called the shop head. Secondly, the shank diameter swells in the hole to fill it entirely. A bucked rivet holds the metal pieces in compression (like your thumb and forefinger holding two pieces of paper together) and in shear (does not allow the pieces of metal to slide around in relationship to each other. Typically multiple rivets are used to hold objects together and the combined strength of all of the rivets have tremendous holding power. Although riveted construction is permanent in nature, it can be easily repaired by drilling the existing rivets out, making any necessary repairs and re-riveting with the same or a slightly larger size of rivet.
Although it will take some time and practice to learn how to do this, it really isn’t a terribly difficult skill to acquire.
First thing is the rivet gun. The rivet gun is like a hand held pneumatic jack hammer for driving rivets and could easily be confused with an air hammer. However, there is a significant difference between these tools as the rivet gun’s impact can be controlled by varying the amount of squeeze on the trigger, whereas an air hammer is on full or off. The gun incorporates a removable coil spring that both holds and retains the rivet set securely to the rivet gun and acts as a return spring each time the rivet gun drives the rivet set outward:
The rivet set is the business end of the rivet gun. You can have one rivet gun and any number of rivet sets to accommodate the shape required for the contour of the manufactured head of the rivets being used. From what I can gather in the plans, the vast majority of bucked rivets in the Zenair aircraft are of the domed head type, so the rivet set I need looks like this:
The bucking bar is the other half of the equation. Just about anything of a hardened material can be used and like the rivets themselves they come in all shapes and sizes, allowing access into tight or blind spaces where required. The brass cylinder on the left in the photo above can be used as a bucking "bar", as can the red cylinder propping up the rivet gun for the photo. Sometimes they are a square block, it doesn't matter as long as it can be held firmly and flat against the tail of the rivet being driven.
Like any fastener, rivets come in any shape and size you desire, and the plans are very specific about the specs required at each location. Usually, the length is measured from the flat underside of the rivet head to the tail and is noted in sixteenths of an inch.
Sometimes, rivets that are too long can be trimmed to the correct length using a rivet cutter. A rivet cutter is like a scissor or shear that can cut rivets of various sizes to specific lengths. The 1/4'” length rivets cannot be cut any shorter with this tool as this the minimum length of cutting. However, the rivet cutter can cut longer rivets (like the 1/2” rivets) to 4/16” (AKA ¼”), 5/16”, 6/16” (or 3/8”) and 7/16”. The rivet cutter has a series of length setting spacers that can be rotated above the selected hole that is appropriate to the rivet diameter being cut. Then, tighten the knurled knob to keep the length setting you desire while cutting a number of rivets:
As with any new skill, practice makes sense. I took a couple of scrap pieces of aluminum, match drilled some holes and got right to bucking some practice rivets.
Because this is a two handed job (sometimes a two person job) it was impossible for me to get pictures of the rivet gun and bucking bar in action, and I forgot to get a picture of the after bucking. I'll post some soon, but I'm happy with my first attempts and Ron says my results are to an acceptable level for construction. Again, it's not a hard skill, but an important one to get right.
Setting aside the rivet gun for a bit, I focused on match drilling and fitting (and of course deburing) the ring root assembly in anticipation of final riveting (both pulled and bucked rivets).
With everything clamped temporarily in place, I noticed that the wing root rib upper flange wasn't matching up properly:
As always, when something doesn't look right, refer to the plans.
The plans sometimes don't describe things as well as they should. Part of that is on me as a new builder. In this case, the plans advise that the flange is only bent to 30 degrees at the front, gently increasing to 90 degrees towards the rear. This makes sense as the wing root skin that it supports curves in the same way. So I'll have to bend it back. Hammering it isn't an option, but using another of Ron's custom made tools is:
With a bit of gentle work and some minor trimming, the root rib now fits as expected:
Next step was figuring out how to drill the rib, spar web, web doubler and nose rib in prep for final riveting.
Much like the wing attach fitting, this involved back drilling the top hole through the web into the rib, cleco, drill the bottom hole, cleco and finally the middle hole, being very careful not to ruin the previous holes:
Remove the root rib, debur and repeat the process for the root nose rib, back drilling from behind the spar. Remove, debur and fit everything together again:
Final riveting of the root ribs will be done once the rear spar fitting is complete and test fitted. I made progress in this regard too by match drilling the rear spar doubler to the rear spar:
As I stated before, the vast majority of rivets used in Zenair aircraft designs are pulled rivets as the sheet metal construction fastener of choice. The spar web doubler was ready for final pulled rivet setting.
The larger A5 rivets are called out in the plans for this assembly. Although they can be pulled by a hand rivet tool, it is much easier and consistent to use the pneumatic rivet gun. It uses the same mechanics as the hand riveter, but is much quicker and completes the "pull" in one shot, alleviating potential issues with partially completed rivets (where the rivet isn't fully pulled before the mandrel breaks off).
The pneumatic rivet gun made short work of setting the A5 rivets:
Backside of spar web doubler - looks like a crop of mushrooms. It's pleasing to see how much rigidity this gives the structure, something ignored by the previous builder. If you look close you can also see the tails of some of the yet to be bucked solid rivets where they poke through the spar cap:
Progress... Bucking and pulling rivets is fun (that opinion may change after doing thousands of them). I'll post some pictures and maybe even some video when I get a chance. Stay tuned :)
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.